Minimize include dependencies of display_server.h

- Removes `native_menu.h` dependency from `display_server.h`.
  It's now forward-declared in all DisplayServer implementations and should
  be included in the .cpp's.
- Removes some unused `rb_set.h` and `rb_map.h` dependencies, which leads to
  having to include them explicitly in half the scene and editor codebase...
  which shows how much we depend on `display_server.h`.
- Forward-declare `input_event.h`, so now we need only `keyboard.h`.

servers/display/native_menu.cpp

@@ -30,7 +30,7 @@
 
 #include "native_menu.h"
 
-#include "scene/resources/image_texture.h"
+#include "scene/resources/texture.h"
 
 NativeMenu *NativeMenu::singleton = nullptr;